Device and method for monitoring an electrical insulation in a vehicle electrical system

ABSTRACT

A device for monitoring an electrical insulation in the case of a vehicle electrical system of a vehicle, includes: a voltage source for generating direct voltages having a first and a second voltage value; a voltage-measuring unit between a positive and a negative power supply line of the vehicle electrical system for measuring a voltage between the positive and the negative power supply line; a current path between a positive current connection of the voltage source and the positive power supply line; a current-measuring unit in the current path for measuring a current flowing through the current path; a determining unit designed to determine an insulation resistance between the vehicle electrical system and an electrical ground from voltage measurement values of the voltage-measuring unit, current measurement values of the current-measuring unit, and the first and the second voltage value.

TECHNICAL FIELD

The invention relates to a device and a method for monitoring anelectrical insulation in an on-board system, specifically of a vehicle,in particular a hybrid/electric vehicle. The invention also relates toan on-board system for a vehicle and a vehicle, specifically ahybrid/electric vehicle with an on-board system, wherein the on-boardsystem includes an above-mentioned device.

BACKGROUND

Modern vehicles, specifically hybrid or electric vehicles, have on-boardsystems incorporating a high voltage (HV) system branch with a servicevoltage of several hundred volts.

However, for a person, specifically a child, a voltage exceeding 60Volts (V) is life threatening. An HV system branch with a servicevoltage exceeding 60 V is therefore electrically insulated from the restof the on-board system or from the vehicle bodywork, in order toeliminate the hazard to persons.

In case of technical defects in the on-board system or operating errors,a “fault current” can flow from the HV system branch through the humanbody. If the electrical insulation between the above-mentioned HV systembranch and the rest of the on-board system or the vehicle bodywork isintact and sufficiently high, the fault current strength is limited to anon-hazardous value for persons.

However, if the electrical insulation is not sufficiently high, a faultcurrent with a rating of several milliamps can flow through the humanbody, which can be life threatening to persons.

As such, in order to avoid such life-threatening fault currents from theoutset, electrical insulation in the on-board system must be constantlymonitored and potential hazards identified at the earliest opportunity.

SUMMARY

Therefore, it is desirable to provide a cost-effective option formonitoring electrical insulation in an on-board system of a vehicle.

The cost-effective option is described by the subjects of the followingdisclosure

According to an aspect of the disclosure, a device is provided formonitoring electrical insulation in an on-board system of a vehicle,specifically a hybrid/electric vehicle.

Accordingly, the device includes a voltage source, which is designed togenerate or deliver a first direct current (DC) voltage with a firstvoltage value and a second DC voltage with a second voltage value.

The device also includes a voltage-measuring unit, which is arranged orelectrically connected between a positive power supply line and anegative power supply line in the on-board system. The voltage-measuringunit is designed to measure a voltage between the positive and thenegative power supply line.

The device may have a current path between a positive power connectionof the voltage source and the positive power supply line in the on-boardsystem. Alternatively, the device may have a current path between thepositive power connection of the voltage source and the negative powersupply line in the on-board system.

The device may also include a current-measuring unit in the current paththat is designed, by the action of the first and the second DC voltagesof the voltage source, to measure a current flowing in the current path.

In some examples, the device includes an evaluation unit, which iselectrically connected via a first signal input to a signal output fromthe voltage-measuring unit and via a second signal input to a signaloutput from the current-measuring unit. The evaluation unit is designedto determine, from a first measured voltage value on thevoltage-measuring unit measured in response to the first DC voltage,from a second measured voltage value on the voltage-measuring unitmeasured in response to the second DC voltage, from a first measuredcurrent value on the current-measuring unit measured in response to thefirst DC voltage, from a second measured current value on thecurrent-measuring unit measured in response to the second DC voltage,and from the first voltage value of the first DC voltage and the secondvoltage value of the second DC voltage, an insulation resistance betweenthe on-board system and electrical ground or between the HV systembranch and the rest of the on-board system.

The term “DC voltage” is understood as a voltage which shows a constantvoltage value for a specific time interval which is used for themeasurement of the above-mentioned measured current values. The voltagesource thus generates DC voltages with different, but constant voltagevalues.

For monitoring the electrical insulation in the on-board system,specifically between the HV system branch and the rest of the on-boardsystem or the vehicle bodywork, the device uses the voltage source togenerate two indirectly or directly sequential DC voltages withdifferent voltage values within a specific time interval. In response tothe two DC voltages, currents with different current values flow in thecurrent path and from the current path to the positive or negative powersupply line on the on-board system, depending upon which of the twopower supply lines the current path electrically connects the voltagesource with.

In some examples, if the electrical insulation in the on-board system,specifically between the HV system branch and the rest of the on-boardsystem or the vehicle bodywork and thus electrical ground issufficiently high, no or virtually no current flows, in response to thetwo DC voltages, from the HV system branch or from the power supplylines to the rest of the on-board system or to electrical ground.

If, for example, as a result of defects in the on-board system, theon-board system or the HV system branch is not adequately insulated fromthe vehicle bodywork or from the rest of the on-board system, a leakagecurrent path is formed between the HV system branch and the rest of theon-board system, or between the power supply lines and the vehiclebodywork, through which, in response to the two DC voltages, leakagecurrents can flow from the HV system branch to the rest of the on-boardsystem or to the vehicle bodywork.

In some implementations, the device uses the current-measuring unit tomeasure currents flowing in the current path, and the voltage-measuringunit to measure voltages between the two power supply lines of the HVsystem branch. In addition, with reference to the measured current andvoltage values and the known voltage values of the two DC voltagesgenerated, the device may determine the insulation resistance betweenthe HV system branch and the rest of the on-board system or the vehiclebodywork.

The determined insulation resistance thus may then be compared with apredefined reference resistance. If the determined insulation resistanceundershoots the reference resistance, an electrical leakage between theon-board system and electrical ground, or between the HV system branchand the rest of the on-board system or electrical ground, may beassumed. Accordingly, warnings are issued to the vehicle driver, orsuitable predefined measures, specifically suitable automatic measures,are deployed. Possible automatic measures include, for example,electrical isolation from the on-board system or the discharging ofelectric energy stores, including e.g. traction batteries orsuper-capacitors, with charging voltages greater than 60V.

As a voltage source, for example, a simple DC voltage converter may beused, which is electrically connected to a LV system branch and which,by stepping-up (and, where applicable, by inverting) the service voltageof the LV system branch generates the two DC voltages. The evaluationunit may include a simple and cost-effective microprocessor and acost-effective analog-digital converter.

In some implementations, the device is thus provided which, using simpleand cost-effective electrical/electronic components, permits for thereliable determination of electrical insulation in an on-board system ofa vehicle, specifically between a HV system branch and the rest of theon-board system or the vehicle bodywork, in a simple manner.

Accordingly, a cost-effective option is provided for the reliablemonitoring of an electrical insulation in an on-board system of avehicle.

In some examples, the device, in the current path, incorporates aresistor for the limitation of currents flowing in the current path inresponse to the two DC voltages. The resistor is thus configured with ahigh ohmic rating. In some examples, the resistor has a resistance valueof the order of one megaohm.

Yet another aspect of the disclosure provides the device that in thecurrent path, incorporates a controllable switch for the interruption orformation of an electrical connection in the current path. By thecontrolled opening of the switch, in the time during which no monitoringof the electrical insulation is in progress, the device may beelectrically isolated from the on-board system, and thus protected frompotential damage by overvoltages. In some examples, when used, i.e. forthe monitoring of the electrical insulation, the device may beelectrically connected to the on-board system, by the controlled closingof the switch.

Yet another aspect of the disclosure provides the device that includingan evaluation unit designed to determine the insulation resistance as aquotient of the difference between the first and the second voltagevalue, corrected respectively by the voltage values observedrespectively on the resistor in the current path in response to therespective DC voltages, and the voltage values of the respectivevoltages associated with the respective DC voltages between the positiveand the negative power supply line, and the difference between the firstand the second current value of the currents flowing in the current pathin response to the respective DC voltages.

Another aspect of the disclosure provides a device having an evaluationunit designed for the determination of the insulation resistance usingthe following equation, by an iterative calculation method:

$R_{{ISO}_{m}} = \frac{\left( {{{Uq}\; 1} - {I\;{1 \cdot R}}} \right) - \left( {{{Uq}\; 2} - {I\;{2 \cdot R}}} \right) - {k_{m} \cdot \left( {{{Ub}\; 1} - {{Ub}\; 2}} \right)}}{\left( {{I\; 1} - {I\; 2}} \right)}$

where:

-   -   Uq1 is the first voltage value of the first DC voltage;    -   Uq2 is the second voltage value of the second DC    -   voltage;    -   I1 is the first measured current value of the current which    -   flows in the current path in response to the first DC voltage;    -   I2 is the second measured current value of the current which        flows in the current path in response to the second DC voltage;    -   Ub1 is the first measured voltage value, measured under the        first DC voltage, of the voltage present between the positive        and the negative power supply line;    -   Ub2 is the second measured voltage value, measured under the        second DC voltage, of the voltage present between    -   the positive and the negative power supply line;    -   R is the electrical resistance in the current path or an        electrical resistance between the positive power terminal of the        voltage source and the positive or    -   negative power supply line of the on-board system; and    -   K_(m) is a corrective value.

In some implementations, the evaluation unit is designed for thedetermination of the corrective value km using the following equation,by an iterative calculation method:

$k_{m} = \frac{{Uq} - {I \cdot R_{{ISO}_{m - 1}}} - {I \cdot R}}{Ub}$where Uq, I and Ub respectively are the first voltage value, the firstmeasured current value and the first measured voltage value, orrespectively the second voltage value, the second measured current valueand the second measured voltage value. R_(ISOm-1) is an insulationresistance value, which is determined in the (m−1)^(th) iteration by theapplication of the previous equation.

The iteration is repeated until the insulation resistance value R_ISOmno longer varies, varies by a negligibly small amount, or varies by arecurring decimal fraction. Thereafter, the most recently determinedinsulation resistance value R_ISOm, as the current insulationresistance, is compared with the predefined reference resistance.

In some implementations, the first voltage value of the first DC voltageis between 50 and 60V. In some examples, the second voltage value ofsecond DC voltage is between −60 and −50V.

The upper limiting values of 60 or −60V thus constitute the upper limitfor generatable DC voltages which are still not life-threatening topersons or vehicle passengers.

The lower limiting values of 50 or −50V permit a voltage range of atleast 100V between the two DC voltages which, in measured current valuesfor the first, second and third currents, is associated withcorrespondingly high differences in value. The high differences inmeasured current values in turn permit the accurate determination ofinsulation resistance. Moreover, high voltage values permit a robustresponse to voltage fluctuations in the current paths or in the on-boardsystem.

A further aspect of the disclosure provides an on-board system for avehicle, specifically for a hybrid/electric vehicle, wherein theon-board system incorporates an above-described device for monitoringelectrical insulation in the on-board system or between an HV systembranch of the on-board system and the rest of the on-board system or thevehicle bodywork.

Yet another aspect of the disclosure provides a vehicle, specifically ahybrid/electric vehicle with an on-board system, wherein the on-boardsystem incorporates an above-described device for the monitoring ofelectrical insulation in the on-board system or between an HV systembranch of the onboard system and the rest of the on-board system or thevehicle bodywork.

Another aspect of the disclosure provides a method for the monitoringelectrical insulation in an on-board system, specifically in a vehicle,specially a hybrid/electric vehicle. The method includes: switching avoltage source between an electrical ground and a power terminal on apositive power supply line of the on-board system or a negative powersupply line of the on-board system; and generating a first DC voltagewith a first voltage value, using the voltage source. The method alsoincludes measuring a first measured voltage value of a voltage betweenthe positive and negative power supply line and a first measured currentvalue of a current flowing between the voltage source and the powerterminal, as the first DC voltage is generated. The method also includesgenerating a second DC voltage with a second voltage value, using thevoltage source; and measuring a second measured voltage value of thevoltage and a second measured current value of the current, as thesecond DC voltage is generated.

The method also includes determining an insulation resistance betweenthe on-board system and electrical ground from the first and the secondvoltage values, the first and the second measured current values and thefirst and the second measured voltage values.

In some examples, a voltage source is switched or electrically connectedbetween electrical ground and a power terminal of a positive powersupply line of the on-board system. In other examples, the voltagesource is switched or electrically connected between electrical groundand a power terminal of a negative power supply line of the on-boardsystem.

For the monitoring of the electrical insulation in the on-board system,a first DC voltage with a first voltage value is generated for aspecific time interval, using the DC voltage source.

During this time, the current flowing in the current path in response tothe first DC voltage and a voltage present between the positive and thenegative power supply line in response to the first DC voltage aremeasured. The first measured current value thus obtained and the firstmeasured voltage value are logged for the determination of an insulationresistance between the on-board system or between a HV system branch ofthe on-board system and the rest of the on-board system or electricalground.

In a given time interval, indirectly or directly after the measurementof the first measured current value and the first measured voltagevalue, a second DC voltage with a second voltage value is generated fora specific time interval, using the DC voltage source.

In some implementations, during this time interval, a current flowing inthe current path in response to the second DC voltage and a voltagepresent between the positive and the negative power supply line inresponse to the second DC voltage are measured. The resulting secondmeasured current value and the second measured voltage value are alsoused in the determination of the insulation resistance.

From the first and the second measured current values, the first and thesecond measured voltage values, and the first and the second voltagevalues of the two DC voltages generated, the insulation resistance isthen determined.

In some examples, the determined insulation resistance value is comparedwith a predefined reference resistance value. If the determinedinsulation resistance value undershoots the reference resistance value,an electrical leakage between the on-board system and electrical ground,or between the HV system branch and the rest of the on-board system orelectrical ground, is assumed.

In some implementations, the above-mentioned steps of generating thefirst DC voltage, measuring the first measure voltage, generating thesecond DC voltage, and determining the insulation resistance areexecuted with the vehicle in service. Thus, electrical insulation in theon-board system may also be determined during the time in which theservice voltage is present in the on-board system and on-board systemcurrents are flowing.

Advantageous implementations and examples of the device described above,insofar as transferable to the above-mentioned on-board system orvehicle, or to the above-mentioned method, are also to be considered asadvantageous aspects of the on-board system, vehicle, or method.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description below. Otheraspects, features, and advantages will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an exemplary on-board system in anelectric vehicle with a device;

FIG. 2 is a schematic view of an exemplary arrangement of operations formonitoring electrical insulation in an on-board system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, an on-board system BN of an electric vehicle, notrepresented in the figure, includes a device V for monitoring electricalinsulation in the on-board system BN. The on-board system BN includes anHV system branch HZ. The HZ system branch carries a service voltage ofapproximately 500V. The HV system branch HZ delivers electric currentfor an electric machine EM in the HV system branch HZ, which is used forthe propulsion of the electric vehicle.

In this HV system branch HZ, the on-board system BN is provided with apower and current source in the form of a traction battery BT, whichdelivers the requisite current for the operation of the electric machineEM or the propulsion of the electric vehicle.

In some examples, by a controllable contactor Sc1, Sc2 in each case, thetraction battery BT is electrically connected respectively to a positiveand a negative power supply line LP, LN of the HV system branch HZ.Additionally to the first contactor Sc1, the traction battery BT may beelectrically connected via a third controllable contactor Sc3 and aprotective resistor Rs to the positive power supply line LP. The thirdcontactor Sc3 and the protective resistor Rs may be electricallyinterconnected in series and electrically connected in parallel to thefirst contactor Sc1 between the traction battery BT and the positivepower supply line LP.

In the HV system branch HZ, the on-board system BN may also include aconverter UR. The converter UR converts the current delivered by thetraction battery BT into phase currents and delivers the phase currentsto the electric machine EM.

The positive power supply line LP and the negative power supply line LNhere connect the traction battery BT to the converter UR, through whichcurrent flows from the traction battery BT to the converter UR.

In some implementations, between the positive power supply line LP andthe negative power supply LN, the HV system branch HZ includes anintermediate circuit capacitor Czk. The intermediate circuit capacitorCzk offsets voltage fluctuations in the service voltage Ub of the HVsystem branch HZ and maintains the service voltage Ub at the requisitevoltage value.

In some examples, between the positive power supply line LP andelectrical ground MA, and between the negative power supply line LN andelectrical ground MA respectively, the HV system branch HZ includes adischarge capacitor Cd.

The on-board system BN may include an LV system branch, not representedin the figure, in which, according to the electric vehicle design, aservice voltage, e.g., of 12V, is present. In this LV system branch,electrical consumers are electrically connected and operate at theservice voltage of 12V. These electrical consumers include, for example,but are not limited to, vehicle lights, navigation systems, electricwindow heaters or electric air-conditioning compressors.

In some examples, at 500V, the service voltage Ub on the HV systembranch HZ is life-threatening to vehicle passengers. In order to preventthe uncontrolled discharge of current in the HV system branch HZ, at thehigh service voltage Ub, via the LV system branch or via the electricalground MA to the on-board system BN or the vehicle bodywork, thusendangering the health of vehicle passengers, the HV system branch HZ orthe two power supply lines LP, LN are galvanically separated andelectrically insulated from the rest of the on-board system BN, and thusfrom the LV system branch or the electrical ground MA.

Electrical insulation between the HV system branch HZ and the rest ofthe on-board system BN, and thus the electrical ground MA, isschematically represented in the figure by the notional insulationresistance R_ISO.

In some implementations, to prevent any flow of current from the HVsystem branch HZ to the LV system branch or to the electrical ground MA,and thus the potential electrocution of persons, the insulationresistance R_ISO may be sufficiently high.

In the event of defects or faults in the on-board system, galvanicisolation between the HV system branch HZ and the rest of the on-boardsystem BN or the electrical ground MA may be compromised, potentiallyresulting in damage to the electric vehicle and injury to vehiclepassengers.

For the improvement of safety in the electric vehicle, electricalinsulation or an insulation resistance between the HV system branch HZand the rest of the on-board system BN or electrical ground MA may becontinuously monitored. In some examples, if the insulation resistancevalue undershoots a stipulated reference resistance value, the tractionbattery BT, by the controlled opening of all three contactors Sc1, Sc2,Sc3, may be electrically isolated from the HV system branch HZ, and theintermediate circuit capacitor Czk may be discharged down to a chargingvoltage of less than 60V.

In some implementations, the on-board system BN includes a device V formonitoring electrical insulation in the on-board system BN, or betweenthe HV system branch HZ and the rest of the on-board system BN orelectrical ground MA.

In some examples, the device V includes a power terminal SP, via whichthe device V or a circuit member of the device V is electricallyconnectable to the positive or the negative power supply line LP, LN ofthe HV system branch HZ. As shown in FIG. 1, the device V or a circuitmember of the device V is electrically connected to the positive powersupply line LP, as designated in the figure by the number 1.Alternatively, the device V may be electrically connected to thenegative power supply line LN, as illustrated in the figure by thenumber 2.

Between the power terminal SP and electrical ground MA, the device Vincludes a voltage source SQ configured as a DC voltage converter with aconnectable/disconnectable inverter. Via a positive power connection AP,the voltage source SQ may be electrically connected to the powerterminal SP of the device V. Via a negative power connection AN, thevoltage source SQ may be electrically connected to the electrical groundMA.

Between the power terminal SP and the positive power connection AP ofthe voltage source SQ, the device V is provided with a current path P,via which the voltage source SQ is electrically connected to thepositive power supply line LP.

In some examples, in the current path P, the device V includes acurrent-measuring unit ME2. The current measuring unit ME2 iselectrically connected between the power terminal SP and the positivepower connection AP of the voltage source SQ.

The device V may also include a coupling resistor R, which iselectrically connected in the current path P between the power terminalSP and the current-measuring unit ME2.

The device V may also include a controllable switch S electricallyconnected in the current path P between the power terminal SP and thecoupling resistor R. In a closed circuit state, the switch S may form anelectrical connection between the positive power supply line LP and thecurrent path P, specifically the voltage source SQ. In an open circuitstate, the switch S may interrupt the electrical connection between thepositive power supply line LP and the current path P, and thus thevoltage source SQ.

In some examples, the device V also includes a control unit SE for theclosing or opening of the switch S.

The device may include a first voltage-measuring unit ME1, which isconnected between the positive and the negative power supply lines LP,LN of the HV system branch HZ. The first voltage-measuring unit ME1 maybe designed for the measurement of the voltage Ub on the positive powersupply line LP in relation to the negative power supply line LN.

Between the positive power connection AP of the voltage source SQ andthe electrical ground MA, the device V may include a voltage divider ST,which is connected in parallel to the voltage source SQ. The voltagedivider ST includes two series-connected resistors R1, R2. Between acentral tap-off point MG of these two resistors R1, R2 and theelectrical ground MA, the device V incorporates a secondvoltage-measuring unit ME3, which is designed for the measurement of thevoltage U2 on the resistor R2.

The device V may also include an evaluation unit EE for thedetermination of the notional insulation resistance R_ISO. Theevaluation unit EE may include a first, second and third signal inputSE1, SE2, SE3. Via the first signal input SE1, the second signal inputSE2 and the third signal input SE3, the evaluation unit EE iselectrically connected to a signal output SA1 on the firstvoltage-measuring unit ME1, to a signal output SA2 on thecurrent-measuring unit ME2, and to a signal output SA3 on the secondvoltage-measuring unit ME3 respectively.

The operating method of the device V, and specifically of the evaluationunit EE, is described in greater detail hereinafter with reference toFIG. 2, in conjunction with the description of the method for monitoringthe electrical insulation in the on-board system BN or in the HV systembranch HZ.

For monitoring the electrical insulation in the on-board system BN, thenotional insulation resistance R_ISO is determined continuously duringthe operation of the electric vehicle and compared with a predefinedreference resistance value.

To this end, the control unit SE, in a process step S100, closes theswitch S, thereby connecting the device V and the voltage source SQ tothe HV system branch HZ, thus forming an electrical connection betweenthe current path P or the device V and the positive power supply lineLP.

In a process step S200, the voltage source SQ generates a first DCvoltage with a first predefined voltage value Uq1 of 60V for apredefined first time interval t1. In so doing, the voltage source SQconverts the 12V service voltage from the LV system branch to 60V.

This first DC voltage generates a DC current I in current path P,flowing from the current path P into the positive power supply line LP.In the positive power supply line LP, the DC current I is superimposedon a service current Ib flowing in the positive power supply line LP,resulting in a voltage variation in the service voltage Ub.

The service voltage Ub, including the voltage variation, is measured bythe first voltage-measuring unit ME1 in a further process step S300, andis used to determine the insulation resistance R_ISO. In so doing, thefirst voltage-measuring unit ME1, at a predefined time interval td1which is temporarily offset from the starting time at which the voltagesource SQ starts to generate the first DC voltage, measures the servicevoltage Ub, and refers a first measured voltage value Ub1 to theevaluation unit EE. This predefined time interval corresponds to thetime interval for the damping of the intermediate circuit capacitor Czkand the discharge capacitors Cd in the on-board system BN following theapplication of the first DC voltage to the positive power supply lineLP.

Simultaneously, the current-measuring unit ME2 may measure the DCcurrent I in current path P and deliver a first measured current valueI1 via the signal output SA1 to the evaluation unit EE.

In addition, and also simultaneously, the second voltage-measuring unitME3 may measure the voltage U2 on the resistor R2 of the voltage sectionST and, via the signal output SA3, and may deliver a first measuredvoltage value U21 of the voltage U2 to the evaluation unit EE.

After the expiry of the first time interval t1 and a predefined timeinterval tv needed for the damping of the intermediate circuit capacitorCzk and the discharge capacitors Cd, the voltage source SQ in a furtherprocess step S400 generates a second DC voltage with a second predefinedvoltage value Uq2 of −60V for a predefined second time interval t2. Inso doing, the voltage source SQ converts the 12V service voltage fromthe LV system branch to 60V, and inverts the latter.

By analogy to process step S300, the current-measuring unit ME2, in afurther process step S500 during the second time interval t2, which istemporarily offset by a predefined time interval td2 from the startingtime at which the voltage source SQ starts to generate the second DCvoltage, measures the current I flowing in current path P in response tothe second DC voltage, and delivers a second measured current value I2to the evaluation unit EE. The predefined time interval td2 maycorrespond to the interval time needed for the damping of theintermediate circuit capacitor Czk and the discharge capacitors Cd inthe on-board system BN following the application of the second DCvoltage to the positive power supply line LP. Simultaneously, the twovoltage-measuring units ME1, ME3 may measure the respective voltages Ub,U2 and may deliver a respective second measured voltage value Ub2, U22to the evaluation unit EE.

In a further process step S600, the evaluation unit EE may thencalculate, from the first and the second measured voltage values U21,U22, using the following two equations (EQ. 1 and EQ. 2), the firstvoltage value Uq1 of the first DC voltage and the second voltage valueUq2 of the second DC voltage, which have actually been generated by thevoltage source SQ in the first and the second time intervals:

$\begin{matrix}{{{Uq}\; 1} = {{\frac{{R\; 1} + {R\; 2}}{R\; 2} \cdot U}\; 21}} & (1) \\{{{Uq}\; 2} = {{\frac{{R\; 1} + {R\; 2}}{R\; 2} \cdot U}\; 22}} & (2)\end{matrix}$

By the measurement of the two measured voltage values U21, U22 and thesubsequent calculation of the two voltage values Uq1, Uq2 for the DCvoltages actually generated, a potential evaluation error in theinsulation resistance associated with a deviation in the DC voltagesgenerated is prevented, and the accuracy of the insulation resistanceR_ISO determined is thus enhanced.

Once the evaluation unit EE has calculated the two voltage values Uq1,Uq2, the evaluation unit EE uses these two voltage values Uq1, Uq2 andthe two measured current values I1, I2, together with the two measuredvoltage values Ub1, Ub2 from the first voltage-measuring unit ME2, forthe iterative determination of the insulation resistance R_ISO_(m),using the following equation (EQ. 3):

$\begin{matrix}{{R\_ ISO}_{m} = \frac{\left( {{{Uq}\; 1} - {I\;{1 \cdot R}}} \right) - \left( {{{Uq}\; 2} - {I\;{2 \cdot R}}} \right) - {k_{m} \cdot \left( {{{Ub}\; 1} - {{Ub}\; 2}} \right)}}{\left( {{I\; 1} - {I\; 2}} \right)}} & (3)\end{matrix}$where k_(m) is a corrective value. This corrective value k_(m) isdetermined iteratively by the evaluation unit EE from the most recentlydetermined insulation resistance R using the following equation (EQ. 4):

$\begin{matrix}{k_{m} = \frac{{Uq} - {I \cdot R_{{ISO}_{m - 1}}} - {I \cdot R}}{Ub}} & (4)\end{matrix}$

The evaluation unit EE repeats the iteration until the insulationresistance value R_ISO_(m) no longer varies, varies by a negligiblysmall amount, or varies by a recurring decimal fraction. Thereafter, themost recently determined insulation resistance value R_ISO_(m) as thecurrent insulation resistance, is compared with the referenceresistance.

If the insulation resistance R_ISO thus determined exceeds the referenceresistance, it is assumed that the electrical insulation in the on-boardsystem BN or between the HV system branch HZ and the rest of theon-board system BN or the electrical ground MA is sufficiently high, andfulfils the corresponding safety requirements.

As soon as the insulation resistance R_ISO thus determined undershootsthe reference resistance, an electrical leakage in the on-board systemBN or between the HV system branch HZ and the rest of the on-boardsystem BN or the electrical ground MA is assumed. Accordingly, thetraction battery BT, by the controlled opening of the three contactorsSc1, Sc2, Sc3, is electrically isolated from the HV system branch HZ andthe intermediate circuit capacitor Czk is discharged to a chargingvoltage below 60V, and further appropriate measures are implemented.

The method described above may also be executed continuously during theoperation of the electric vehicle, during which the traction battery BT,via the two power supply lines LP, LN, supplies current to the converterUR or the electric machine EM.

In some examples, during the respective time intervals t1, t2, aplurality of measured current and voltage values for the DC current Iand the respective voltages Ub, U2 may be measured in sequence and, fromthis plurality of measured values, average values can be generated forthe respective measured current values I1, I2 and the respectivemeasured voltage values Ub1, Ub2, U21, U22. By this arrangement,potential disturbances such as current fluctuations in the on-boardsystem BN can be filtered out, thereby enhancing the accuracy of theinsulation resistance R_ISO thus determined.

Once determination is complete, the control unit SE may open the switchS, thereby separating the device V from the on-board system BN or fromthe HV system branch HZ.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A device for monitoring an electrical insulationin an on-board system, the device comprising: a voltage source forgenerating a first DC voltage with a first voltage value and a second DCvoltage with a second voltage value; a voltage-measuring unit between apositive power supply line of the on-board system and a negative powersupply line of the on-board system for the measurement of a voltagebetween the positive power supply line and the negative power supplyline; a current path between a positive power connection of the voltagesource and the positive or the negative power supply line; acurrent-measuring unit in the current path for the measurement of acurrent flowing in the current path; and an evaluation unit electricallyconnected via a first signal input to a signal output from thevoltage-measuring unit and via a second signal input to a signal outputfrom the current-measuring unit, the evaluation unit designed todetermine, from a first measured voltage value measured in response tothe first DC voltage and from a second measured voltage value measuredin response to the second DC voltage on the voltage-measuring unit, froma first measured current value measured in response to the first DCvoltage and a second measured current value measured in response to thesecond DC voltage on the current-measuring unit, and from the firstvoltage value and the second voltage value, an insulation resistancebetween the on-board system and electrical ground.
 2. The device ofclaim 1, further comprising a resistor in the current path for limitingthe current.
 3. The device of claim 1, further comprising a controllableswitch in the current path for the interruption or formation of anelectrical connection in the current path.
 4. The device of claim 1,wherein the evaluation unit is designed to determine the insulationresistance as a quotient of the difference between the first voltagevalue and the second voltage value, corrected respectively by voltagevalues observed respectively on a resistor in the current path, and thevoltage values of the respective voltages between the positive powersupply line and the negative power supply line, and the differencebetween the first measured current value and the second measured currentvalue.
 5. The device of claim 1, wherein the evaluation unit is designedfor the iterative determination of the insulation resistance using thefollowing equation:${R\_ ISO}_{m} = \frac{\left( {{{Uq}\; 1} - {I\;{1 \cdot R}}} \right) - \left( {{{Uq}\; 2} - {I\;{2 \cdot R}}} \right) - {k_{m} \cdot \left( {{{Ub}\; 1} - {{Ub}\; 2}} \right)}}{\left( {{I\; 1} - {I\; 2}} \right)}$where R_(ISO) _(m) is the insulation resistance, Uq1 is the firstvoltage value of the first DC voltage, Uq2 is the second voltage valueof the second DC voltage, I1 is the first measured current value, I2 isthe second measured current value, Ub1 is the first measured voltagevalue, Ub2 is the second measured voltage value, R is an electricalresistance of the resistor, and k_(m) is a corrective value.
 6. Thedevice of claim 5, wherein the evaluation unit is designed for theiterative determination of the corrective value k_(m) using thefollowing equation:$k_{m} = \frac{{Uq} - {I \cdot R_{{ISO}_{m - 1}}} - {I \cdot R}}{Ub}$where Uq, I and Ub respectively are the first voltage value, the firstmeasured current value and the first measured voltage value, orrespectively the second voltage value, the second measured current valueand the second measured voltage value, and R_(ISOm-1) is an insulationresistance value, which is determined in the (m−1)^(th) iteration, wherem≥1.
 7. The device of claim 1, wherein the first voltage value is 50 to60V and the second voltage value is −60 to −50V.
 8. An on-board systemfor a vehicle, wherein the on-board system incorporates a device asclaimed in claim 1 for the monitoring of electrical insulation in theon-board system.
 9. A method for monitoring electrical insulation in anon-board system of a vehicle, the method comprising: switching a voltagesource between an electrical ground and a power terminal on a positivepower supply line of the on-board system or a negative power supply lineof the on-board system; generating a first DC voltage with a firstvoltage value, using the voltage source; measuring a first measuredvoltage value of a voltage between the positive power supply and thenegative power supply line and a first measured current value of acurrent flowing between the voltage source and the power terminal, asthe first DC voltage is generated; generating a second DC voltage with asecond voltage value, using the voltage source; measuring a secondmeasured voltage value of the voltage and a second measured currentvalue of the current, as the second DC voltage is generated; determiningan insulation resistance between the on-board system and electricalground from the first and the second voltage values, the first and thesecond measured current values and the first and the second measuredvoltage values.
 10. The method of claim 9, wherein generating the firstDC voltage, measuring the first measured voltage value, generating thesecond DC voltage, measuring the second measured voltage, anddetermining the insulation resistance are executed while the vehicle isin service.